Bill Ackman has threatened to liquidate his 18-percent stake in the struggling retailer if the board doesn’t step up its search to replace interim CEO Mike Ullman.REUTERS

As he ratchets up his war of words with JCPenney’s board, Bill Ackman is putting his money where his mouth is.

As reported by The Post today, the hedge-fund billionaire has threatened to liquidate his 18-percent stake in the struggling retailer if the board doesn’t step up its search to replace interim CEO Mike Ullman.

Likewise, The Post has learned that Ackman is prepared to back Penney with fresh financing if a new CEO is named in the coming weeks.

“Bill has been totally sidelined by the board and cut off from information, and he says he can’t remain an investor under those conditions,” according to a source close to the situation.

“If he gets his way, of course, it’s a different story,” the source said of Ackman, who sent a lengthy e-mail to Penney’s board today, following a pair of testy letters during the past week.

Ackman made his initial threat to sell his Penney stock at a July 22 board meeting, sources said.

Ackman is using his cash as a cudgel despite public pledges earlier this year to stick with his Penney investment until the company recovers from a disastrous turnaround bid by former CEO Ron Johnson.

Penney blasted Ackman yesterday, noting that he had spearheaded the hire of Johnson, whose move to eliminate coupons and sales events spurred a $1 billion loss last year and sent sales tumbling 25 percent.

The board also said it “strongly disagrees” that the struggling retailer needs a new CEO.

Nevertheless, Ackman today won the public support of fellow hedge-fund tycoon Richard Perry, who will back the hire of Foot Locker CEO Ken Hicks to lead Penney, with former Penney CEO Allen Questrom as chairman, according to CNBC.

Questrom yesterday said he would consider becoming chairman if the right CEO is found.

Penney shares have see-sawed as the boardroom brawl escalates. The stock was recently down 7.6 percent, erasing a rally yesterday that was spurred by Ackman’s public demand for a new CEO.

In his latest letter today, Ackman accused Ullman of cutting off his hedge fund Pershing Square Capital Management, as well as Blackstone Group, from the retailer’s financial data in recent months.

Ackman said Ullman has also fired consulting firm Alix Partners, which was brought in to help manage the company’s cash.

“Material hiring and firing decisions are being made without the board being properly consulted,” Ackman said, blasting Ullman’s hire of former Kraft exec Debra Berman as Penney’s new marketing chief.

“Imagine my surprise when I learned of Ms. Berman’s hiring from a press release on my Bloomberg machine,” Ackman wrote.

Ackman also blasted Ullman for keeping a pair of Gulfstream G450 jets, which famously had been used by Johnson for weekly commutes to his home in California.

“One has to ask the appropriateness of our aircraft fleet in light of the current state of the company,” Ackman wrote.